A tool or a workpiece, or a machine tool itself may be damaged or broken, and worst of all, an operator may be injured, due to extraordinary collision of a tool with a workpiece which may be produced by mistakes in operation during machining process in such machine tools as a grinding machine, lathe, milling machine, planer, and drilling machine.
To provide countermeasures against above unusual collision, a preventing means in the prior art is generally effected by detecting fluctuation of the comsumption power of an electric motor for a wheel shaft in case of a grinding machine. However, this detecting method is not sensitive. And it has been also known in the prior art that the feedrate of a tool is changed by taking out specified oscillation waves produced by collision by means of strain gauge, converting said oscillating waves into electrical signals, selecting said electrical signals and amplifying them. But a device for taking out specified oscillating waves becomes complicated and it has a critical defect that it is not sensitive for other than said specified oscillating waves. Still more it is proposed in order to improve such a previous device that a device can detect wide range of frequency. Actually however, it takes at least 100 m. seconds to issue a detection signal in order to effect rectification smoothing.